This project involves theoretically grounded research on the co-occurrence of internalizing and externalizing behaviors among children, with a focus on demographic disparities in such co-occurrence. Doing so in a nationally representative framework will inject much-needed awareness of population heterogeneity into theoretical perspectives on developmental psychopathology. Accomplishing the aims of this proposal will help inform educational and health policy by providing research-based evidence about those most in need of intervention (defined by race/ethnicity, gender, and immigration status), and when (timing), where (mechanism and buffering), and how such intervention may be delivered for maximum effectiveness. The project aims will be accomplished by using the Early Childhood Longitudinal Study-Kindergarten Cohort and a wide range of longitudinal quantitative methods, including growth curve modeling, mediational models, and logistic regression. Furthermore, in an effort to tackle the selection problems inherent in non-experimental studies of health, propensity scores will be coupled with post-hoc robustness indices to quantify the vulnerability of results to observable and observable confounds. More specifically, this project will identify the prevalence and timing of childhood trajectories of internalizing and externalizing behaviors, investigate gender, race/ethnic, and immigration-related disparities in the over-time patterns and timing of children's co-occurring behaviors, examine how such co-occurrence is connected to problematic family processes in different demographic groups at different timepoints, and identify school settings that may buffer against family risks contributing to the co-occurrence of the two behaviors across groups. To pursue these aims, the investigator will work under the guidance of a sociologist in an NICHD-funded population center and a trained clinical child psychologist. By identifying critical intervention points for young children's co-occurring behavioral and emotional problems that can have long-term consequences for health and other life course trajectories, determining which segments of the population are at heightened risk, and connecting family problems leading to child maladjustment with potentially compensatory school resources already subject to established policy interventions, this project will promote finer-grained prevention and intervention programs. At the same time, it will give valuable interdisciplinary training to a promising young scholar with a future in population health and developmental research.